1. Field of the Invention
This invention relates to a method of controlling a grinder robot. In more detail, the present invention relates to a method of controlling a grinder robot wherein a grinder is mounted at a tip of a robot finger, and a grinding operation is performed on a work surface while controlling a pressing force of the grinder with respect to the work surface.
2. Description of The Background Art
Grinder robots are known in which a grinding tool such as a grinder is mounted at a tip of a robot finger, and a grinding operation is performed while pressing this grinder against a work of an arbitrary shape with a prescribed force. Specific examples of such grinder robots are illustrated in U.S. Pat. No. 5,126,645 and U.S. Pat. No. 5,265,195, etc.
In a grinding operation performed by such a grinder robot, the pressing force of the grinder against the work is detected by a six-axis force/torque sensor mounted at the tip of the robot arm, and the target position and attitude of the grinder are successively corrected while successively comparing this detected pressing force with a target pressing force, thereby controlling the target pressing force so that it is maintained constant.
Thus, while using such a control method, for example as shown in FIG. 1, the grinder is progressively fed in the X direction while performing a plurality of reciprocations over the grinding region of the work. As a result, a smooth surface condition can be produced over the grinding region.
However, in a conventional grinder robot controlled as described above, in cases where a grinding operation is performed that has to grind a work surface in which a comparatively large number of burrs and/or irregularities are present before grinding, there is a high probability of the following problems occurring.
The first problem is that, when a work surface in which comparatively large irregularities are present has to be ground smooth, a step (groove) is formed along the boundary of the grinding region on the work surface after grinding.
Formation of such a step risks giving rise to stress localization at the step portion. If a large force is applied to such a work, stress concentration occurs in the vicinity of the step, leading to the formation of cracks from the vicinity of the step.
Furthermore, since the step formed by the conventional grinding operation is formed at right angles when seen in the cross-sectional plane of the work, it is fairly sharp, and thereby presents a risk of scratching other works, etc., coming into proximity with it. In particular, such steps are often formed in a straight line along the grinding boundary.
A second problem is that, when grinding smooth comparatively large burrs and/or irregularities present in the work surface, as shown in FIG. 1, the grinder 11 may be damaged when moving in the A direction.
Specifically, whereas, when grinder 11 is moved in the B direction (retracted), the edge of the grindstone is dragged over the surface of the work. When grinder 11 is moved in the A direction (advanced), however, the edge of the grindstone moves in such a way that it can snag on the work. In this latter case therefore it sometimes happens that the presence of comparatively large burrs and/or irregularities in the work surface make grinding of the burrs and/or irregularities impossible, because they prevent movement of the grindstone. In some cases there is even a risk of the grindstone and work, or the grinder robot itself, being damaged.
Consequently, in order to avoid this problem, the method was adopted of only performing the grinding operation by pressing the grindstone against the work while the grinder is being retracted in direction B; whereas while the grinder is being advanced in direction A, it is moved up to the line Y=0 through the space above the work without performing a grinding operation. However, with the method in which the grinder is moved in the direction A through the space above the work, difficulty is experienced in accurately guiding and returning the grinder to the correct position for recommencement of the grinding operation. This is because it is not possible to make use of force control, since when the grinder is being guided and returned, grinding is not being performed.
Even if the grinder were to be guided to a point on the boundary (Y=0, Z&gt;0) of the grinding region using position control, usually, due to variability of the position of fixing of the individual work, the relative position of the work with respect to the grinder robot would not always be fixed. There would therefore be a risk of the grinder being guided too far away from the work, or alternatively, of the grindstone of the guided grinder colliding with the work, and thereby damaging the work.